In recent developments of telecommunications technology, in particular, and in the development of broadband integrated services networks (B-ISDN), the asynchronous transfer mode (ATM) based on asynchronous time-division multiplex technique plays a significant part. The signal transmission is consequently undertaken in a bit stream that is subdivided into cells, each respectively composed of header and useful information parts, of a constant length of, for example, 53 octets that are occupied as needed with packeted messages. Special cells are transmitted when no useful information is to be communicated. Virtual connections, i.e. connections that only in fact use a path section when a message packet (block) is in fact to be communicated via the path section are set up in ATM switching centers, whereby every packet contains, among other things, an address covering, for example, two octets in its header for the unambiguous allocation of the packet to a specific virtual connection. Based on the measure of the respective selection information, every packet at the input to the switching network can thereby contain the complete information for its route through the switching network. Using this information, the switching elements themselves then switch the packet on the defined route (self-routing network) (see, for example, Telcom Report 11 (1988) 6, 210... 213). A two-stage reversing switching network having (32.times.32) switching matrices can thereby be provided as the switching network, these switching matrices being in turn each respectively capable of being formed of four switching pyramids formed with (16.times.8) switching elements (see, for example, International Switching Symposium 1900 "Innovations in Switching Technology", Stockholm, Sweden, May 28-Jun. 1, 1990. FIG. 10). For a greater plurality of input ports or output ports to be connected, a stretched three-stage switching network will be provided instead. Since switching equipment must guarantee a practically interruption-free switching operation, at least two redundant switching levels will be provided, i.e. the switching network will be doubled (see, for example, European Patent Application No. 0 384 936).
When establishing switching centers, the case is often is such that, given what is initially a limited number of subscribers, a fully expanded switching network is not initially required. On the contrary, only a partially expanded switching network is adequate, which is capable of being later expanded to form a fully expanded switching network. In order to be able to undertake such an expansion without interrupting operations, a maximally configured, middle switching stage is usually provided from the very outset in a two-stage reversing switching network even when this only has a partial configuration level. For expansion, switching matrices in the first and in the third switching stage are then merely added until the full configuration is achieved and are connected via corresponding links to the switching matrices of the middle switching stage. The switching matrices of the middle switching stage that are already provided in a partial configuration of the switching network, but that are not yet required, then represent an unutilized dedicate resource.